1. Field of the Invention
The present invention generally relates to the field of implantable medical devices. More specifically, the present invention relates to an implantable lead for delivering electrical stimuli to a human heart, the lead having a proximal end for connection to an implantable medical device, and a distal end for fixation in the myocardium of a human heart.
2. Description of the Prior Art
Within the field of implantable heart stimulators, such as pacemakers, implantable leads are used for conveying electrical stimuli from the heart stimulator to the myocardium of a human heart. Before fixation of an implantable lead to a location in myocardial tissue of a human heart, the physician is interested in knowing whether the selected location is suitable for delivering electrical stimuli and possibly also for sensing electric intrinsic activity, i.e. electrical activity of the heart which is not induced by stimulation pulses from the medical implant. In other words, the physician must know whether delivered electrical stimuli at the selected position will provide a desirable response in the myocardial tissue, and will measurements of intrinsic activity provide a sufficiently strong signal at the selected position? This is conventionally performed by fixating the distal end of the implantable lead to the tissue, sensing intrinsic cardiac activity and/or delivering stimulation pulses and monitoring the response of the heart, i.e. the evoked response. If the results are not satisfactory, the lead could be disengaged from the myocardial tissue and repositioned to another location within the heart. However, this procedure may be time-consuming and cause unnecessary trauma to the endocardial and myocardial tissue.
Recently, a method of finding a suitable location has been developed in which the distal end of an implantable lead is arranged with an electrode surface intended for delivering electrical stimuli to the heart and intrinsic cardiac activity is sensed without the need of fixating the electrode in the myocardial tissue. In this prior art implantable lead, the distal end of the lead is provided with an electrode end surface for contacting endocardial or myocardial tissue. Thereby, a stimulating electrode, such as an extendible and retractable helix, could remain in the retracted state, with no or very little contact with myocardial tissue, while the electrode end surface was used for contacting myocardial tissue for the purpose of locating a suitable fixation location in the heart. In practice, the stimulating electrode is provided with a movable portion for fixation of the lead, for contacting and for delivering stimulating pulses to the myocardium after implantation, and a stationary portion for contacting and delivering stimulation pulses during implantation. Commonly, the movable portion is an extendible and retractable helix arranged within the distal end of the lead and extendible therefrom, and the stationary portion is arranged annularly at the distal end of the lead encircling the helix.
As a consequence, no retraction of the helix and repositioning of the distal end of the lead was required for finding a suitable fixation position. However, the presence of an electrode surface intended for use when finding a suitable fixation location has been observed to have an impact on the energy consumption of a medical implant.